Isaiah Kalebu -- accused of killing a South Park woman after sexually assaulting her and her partner for hours -- has been returned to a state mental hospital for evaluation.

Facing rape and aggravated murder charges in the July 19, 2009, attack of Teresa Butz, Kalebu will remain at Western State Hospital for two weeks while doctors there attempt to determine whether he is competent to stand trial, according to a King County Prosecutor's Office statement Tuesday.

Charged with aggravated first-degree murder 10 days after the attack, Kalebu has a documented history of mental illness.

What role the affliction might have played in the early morning attack on two strangers, though, remains at issue, as prosecutors pursue the charge against him. King County Prosecutor Dan Satterbergdeclined to seek a death sentence in the case, citing concerns that Kalebu's mental state may prevent jurors from ordering execution.

Kalebu is scheduled to appear before King County Superior Court Judge Michael Hayden on Sept. 28, when Hayden will hear argument as to whether Kalebu is able to understand the charges against him and participate in his own defense.

Regardless of that hearing's outcome, Kalebu's ability to stand trial -- a separate issue from his mental state at the time of the crime -- will likely remain in dispute until the case is resolved.

Prosecutors claim Kalebu broke into the South Rose Street home, then spent three hours sexually assaulting Butz and her girlfriend. Satterberg said previously that Butz, 39, died protecting her partner.

According to charging documents, the surviving victim told police she and Butz were sleeping when, at about 1:30 a.m., they awoke to find Kalebu standing over their bed. He was naked, she told police, and armed with a butcher's knife.

"I won't hurt you," Kalebu allegedly told the woman, according to court documents. He said all he wanted was sex.

During the 90 minutes that followed, police contend that Kalebu sexually assaulted each of the women several times. As he did so, police say, the physical attacks on the women intensified as he began cutting more aggressively on the necks of both women.

The surviving victim began to lose large amounts of blood. Concerned, Butz rushed Kalebu, then broke out a bedroom window with a nightstand, police claim.

Mortally injured, Butz dove through the window and her girlfriend ran from the room, police say in court documents. Officers contend Kalebu then collected his clothing and ran from the home.

"Her strength in battling her attacker saved the life of her partner," Satterberg said previously.

According to police, Kalebu escaped and eluded the police until he was arrested at Magnuson Park days later. Prosecutors maintain that DNA and fingerprint evidence collected at the scene matches Kalebu.

Prosecutors have said the attack appears to have been entirely random. No evidence has been found indicating that Kalebu specifically targeted the couple; he may have simply been looking for an open window.

In addition to the aggravated murder charge, Kalebu has been charged with first-degree attempted murder, first-degree rape and first-degree burglary. The only punishments for aggravated murder under state law are death and life in prison without parole.